Transport of CO2 across the alveolar-capillary barrier of dog lungs and CO2 transport in dog muscle are being studied. The role of carbonic anhydrase in the transport mechanism is being determined and a general formulation of CO2 diffusive transport in tissues developed. Data have been obtained by measuring 14CO2 diffusion coefficients for flat portions of dog lung lobes, and by calculating CO2 diffusing capacities for intact dog lung lobes from 14CO2 washout. The function of carbonic anhydrase was deduced from measurements before and after inhibition of the enzyme. For purposes of comparison with less water soluble gases, many experiments include comparable 14CO diffusion measurements. CO2 diffusion measurements in muscle are conducted similar to those made in lungs. In addition, arterial tracer injections, incorporating a vascular compartment reference tracer, will be used to determine flux of 14CO2 between tissue and circulation. The muscle CO2 transport studies are now being initiated, the lung measurements are effectively completed.